The majority of air conditioning systems reject the heat absorbed by the refrigeration cycle through an air cooler heat exchange process. A fan forces outside air through a finned tube heat exchange coil called an air cooled condenser, to cool the hot refrigerant gas or a hot liquid heat transfer medium such as water or an anti-freeze solution. The air-cooler condenser/heat exchanger has many benefits including:
1. Low cost construction, PA1 2. Reliability during freezing weather, PA1 3. A low profile that does not extend above a roofline, and PA1 4. Minimal maintenance requirements. PA1 1. The system is only active during the hot weather; thus, standing water in piping is eliminated during the entire freezing weather season. PA1 2. Water quantities required are a small fraction of those needed with all evaporative systems. PA1 3. Water pressures required are low enough to retain the use of conventional piping systems and materials. PA1 4. Pressures required are low enough to retain the use of centrifugal pumps that eliminate many moving parts, which are single points of failure. PA1 5. The system eliminates the use of compressed air to assist with the atomization of the water. Air compression is energy and maintenance intensive. PA1 6. The loss of city make-up water retains the use of the dry cooler to provide some cooling effect until make-up water is restores. PA1 7. The indirect evaporative process occurs within a short distance 18"-36" (500 mm-1,000 mm) providing the opportunity to retrofit existing air-cooled systems. The system is compact and can provide benefit to both new installations, as well as being retrofit to existing heat rejection systems. PA1 8. The architectural rooflines of buildings are preserved. PA1 9. The energy efficiency of the heat rejection system is significantly improved.
A popular version of air cooled refrigeration systems, used extensively in data centers and industrial process environments, adds an additional heat exchange by absorbing refrigerant heat into circulating brine and then cooling the brine in an outside air cooled coil called a "dry cooler." The circulation of brine overcomes many piping design obstacles that are imposed by air-cooled condensers but the additional heat exchange imposes efficiency losses that are exacerbated by hot weather. These systems become unstable and inefficient, for cooling. Energy efficiency, capacity and reliability diminish as outside air temperature increases. The additional wear and tear reduces the life expectancy of an air-cooled system to as much as 50% of that expected from an evaporative cooled system.
To increase capacity, reliability and energy efficient during hot weather, evaporative cooling processes have been developed to assist the refrigeration process. The evaporative process is more efficient during hot weather because the evaporation of moisture into hot outside air reduces the dry air temperature as much as 20.degree. F. The reduced temperature air causes greater heat exchange in the coils, lowers energy consumption and increases the stability, reliability and longevity of the refrigeration system. A typical evaporative system will reduce outside air temperature from 95.degree. F. to 75.degree. F. The trade-off for these benefits is that all evaporative cooling systems; cooling towers, evaporative condensers and evaporative industrial fluid coolers are maintenance intensive and susceptible to catastrophic freezing. Maintenance programs must be implemented to counteract the effects of corrosion, biological fouling, mud accumulation, etc. Poisons are added to the evaporative water sumps to inhibit both corrosion and biological growth. These chemicals when drained into municipal piping systems cause environmental problems. Many municipalities have banned these chemicals to protect their water systems, further complicating the maintenance of the evaporative systems. Additionally, while these evaporative systems provide excellent stability for refrigeration systems by lowering the temperature of outside air, they are the primary source for propagating deadly microorganisms such as Legionellae pneumophilia, the source of Legionnaire's Disease. Local architectural codes preclude the use of evaporative systems because they are tall and extend above the roofline of buildings. Their large evaporative steam plumes look very industrial and drift from the plumes has been known to infect individuals downstream with Legionnaire's Disease. Additionally, the continuous availability requirements of some businesses require heat rejection during freezing weather. Traditional evaporative equipment exposes large surfaces, pipe and basins full of water to catastrophic freezing. The risks to high reliability facilities such as hospitals, data centers or manufacturers are obvious.
An additional risk to continuous availability type facilities from evaporative systems is the high quality of water that is required for all evaporative cooling. If the municipal services providing make-up water are interrupted, these facilities will shut down.